1. Field of Applicable Technology
The present invention relates to an AC-to-AC power inverter apparatus (referred to in the following simply as an AC inverter apparatus) for converting an AC power source voltage to an output AC power supply voltage whose amplitude can be controlled.
In particular, the invention relates to an AC power inverter apparatus in which the AC power source voltage is first converted to a DC voltage which is supplied to a DC-to-AC inverter circuit, but whereby it becomes unnecessary to utilize a large-value smoothing capacitor for eliminating AC ripple components of the DC voltage.
2. Prior Art Technology
With prior art types of AC inverter apparatus, in which a pulse width modulation (hereinafter referred to as PWM) method is generally utilized for controlling the effective amplitude of the output AC voltage, it is necessary to provide a large-capacity electrolytic capacitor connected across the DC voltage in order to smooth out the AC ripple components of that voltage. However the use of such a capacitor results in various disadvantages, as follows:
(a) A large-capacitance electrolytic capacitor which must withstand a high level of DC voltage has a relatively limited operating lifetime. Thus, the effective operating lifetime of the AC inverter apparatus as a whole will in general be determined by that of the smoothing capacitor. PA1 (b) Breakdown of such a capacitor can cause serious damage to other components of the AC inverter apparatus, since leakage of corrosive electrolyte may occur, or the capacitor may even explode. PA1 (c) It may be necessary to also use a large-value inductive reactor in conjunction with such a capacitor. As a result, the overall size and weight of the AC power inverter apparatus are increased. If such a reactor is not utilized, i.e. the rectified DC voltage is applied directly from the rectifier section across the terminals of the smoothing capacitor, then excessively large values of out-of-phase current may flow (as AC components superimposed on the DC voltage), causing the power factor to become poor. This makes it difficult to achieve power saving by improving the power factor of the inverter apparatus. PA1 (d) It is necessary for the rectifier circuit components to have sufficient drive capability for providing the high levels of current which are required to charge the smoothing capacitor at each time of connecting the AC power source to the AC inverter apparatus. Such a current drive capability is required only at the time of power switch-on, being unnecessary during normal operation of the AC inverter apparatus. PA1 (e) It is necessary to provide some means for discharging the smoothing capacitor after power to the AC inverter apparatus is switched off. This is because the capacitor has a large value of capacitance and will remain charged to a dangerously high voltage for a substantial time after power is switched off, unless such discharging means are provided. PA1 (f) The smoothing capacitor occupies a relatively large amount of space within the AC inverter apparatus, thereby reducing the freedom available for mechanical design of the apparatus, and causing the overall size of the AC inverter apparatus to be large. PA1 AC-to-DC converter means for converting the input AC voltage to a DC voltage having AC ripple components; PA1 amplitude detection means for deriving an amplitude detection signal, the amplitude detection signal varying in accordance with changes in amplitude of the DC voltage; PA1 reference waveform generating means for generating a reference waveform signal, the reference waveform signal varying in accordance with changes in amplitude of a reference waveform; PA1 operational means for operating on the reference waveform signal and amplitude detection signal to obtain a ratio signal, the ratio signal varying in amplitude in accordance with a ratio of the amplitude of the reference waveform to the amplitude of the DC voltage; PA1 pulse width modulation signal generating means for operating on the ratio signal to generate a pulse width modulation signal, the pulse width modulation signal varying in duty factor in accordance with changes in the amplitude of the ratio signal; and PA1 A DC-to-AC inverter circuit, controlled by the pulse width modulation signal for converting the DC voltage to the output AC voltage; PA1 the DC-to-AC inverter circuit being controlled by the pulse width modulation signal such as to compensate the amplitude of the output AC voltage for changes in the DC voltage amplitude, to thereby compensate against the ripple components. PA1 triangular waveform generating means for generating a triangular waveform signal having a fixed period; and PA1 a comparator circuit for comparing the triangular waveform signal with the ratio signal, to obtain the pulse width modulation signal. PA1 AC-to-DC converter means for converting the input AC voltage to a DC voltage having AC ripple components; PA1 amplitude detection means for deriving an amplitude detection signal, the amplitude detection signal varying in accordance with changes in amplitude of the DC voltage; PA1 analog-to-digital converter means for converting the amplitude detection signal to digital data; PA1 a DC-to-AC inverter circuit formed of a plurality of controllable switching elements, for converting the DC voltage to the output 3-phase AC voltage, and supplying respective phases thereof to three output terminals; PA1 reference vector memory means having stored therein data representing a reference pattern of vectors, the pattern corresponding to a reference waveform of the output 3-phase AC voltage, the vectors including unit length vectors and zero length vectors, each zero length vector representing a condition of zero potential difference between respective phases of the output 3-phase AC voltage; PA1 a microcomputer coupled to take in, at successive time points recurring with a fixed period, a detected digital amplitude value from the analog-to-digital converter means, a preset amplitude factor, and a unit length vector read out from the reference vector memory means, for producing during a first portion of the fixed period a combination of control signals in accordance with the unit length vector, for then reading out from the reference vector memory means a zero length vector and producing during a second portion of the fixed period a combination of control signals in accordance with the zero length vector, and for determining the ratio of the first and second portions in accordance with a ratio of the preset amplitude factor to the detected amplitude value; and PA1 a drive circuit coupled to the DC-to-AC inverter circuit and responsive to each combination of control signals for setting the plurality of switching elements to a combination of states in accordance with the combination of control signals; PA1 the microcomputer functioning during each of the fixed periods to increase the second time period portion in relation to the first portion in accordance with an increase in the DC voltage amplitude as expressed by the detected amplitude value, for thereby compensating the output 3-phase AC voltage against the ripple components. PA1 a plurality of main rectifier diodes interconnected to form a main rectifier circuit, for transferring power from the AC power source to the DC-to-AC inverter circuit by forward current flow through the rectifier diodes; PA1 a plurality of current transfer elements each connected in parallel with a corresponding one of the rectifier diodes; and PA1 synchronizing means operating in synchronism with the input AC voltage from the AC power source, for controlling each of the current transfer elements to selectively permit transfer of power from the DC-to-AC inverter means to the AC power source by current flow through the current transfer elements in the inverse direction to the forward current flow. PA1 (a) obtaining a detected instantaneous amplitude value proportional to the DC voltage; PA1 (b) reading out, from a memory, one of a sequence of stored reference waveform instantaneous amplitude values; PA1 (c) obtaining a preset amplitude factor indicative of a desired amplitude of the output AC voltage; PA1 (d) calculating the product of the reference waveform amplitude value and the amplitude factor, and dividing the product by the detected amplitude value (Er) to obtain a ratio value; PA1 (e) comparing the ratio value with a current count value of a counter whose count state is successively incremented and successively decremented in alternating intervals, to obtain a comparison value; and PA1 (f) latching the comparison value in a latch circuit to thereby obtain the pulse width modulation signal as an output signal from the latch circuit, with the pulse width modulation signal having a duty factor which decreases in accordance with an increase in the DC voltage resulting from the ripple components. PA1 (a) obtaining a detected amplitude value proportional to the DC voltage; PA1 (b) obtaining data representing one unit length vector of a reference pattern of vectors, the reference pattern expressing a reference waveform of the output 3-phase AC voltage; PA1 (c) obtaining a preset amplitude factor indicative of a desired amplitude of the output 3-phase AC voltage; PA1 (d) generating a combination of drive signals determined in accordance with the unit length vector, during a first portion of the fixed period, determining the duration of the first portion in proportion to the ratio of the amplitude factor to the detected amplitude value, and supplying the drive signals to establish a corresponding combination of states of the switching elements during the first portion of the fixed period; PA1 (e) obtaining data representing a zero length vector, corresponding to a condition of zero potential difference between respective phases of the output 3-phase voltage; and PA1 (f) generating a combination of drive signals determined in accordance with the zero length vector, and supplying the drive signals to establish a corresponding combination of states of the switching elements during a remaining portion of the fixed period.